1. Field of the Invention
The present invention relates generally to a transmission of a vehicle, such as a riding mower and more specifically relates to a transmission distributing power between left and right stepless transmission mechanisms for respective left and right drive axles.
2. Related Art
As disclosed in Japanese Laid-Open Gazette No. 2000-71790, there is a well-known conventional vehicle transmission for a working vehicle such as a riding lawn mower, which has a pair of HSTs for driving respective drive axles and a PTO shaft for driving a working machine attached to the vehicle. The transmission is provided with a pair of left and right HSTs for respective left and right drive axles, a center casing disposed between HSTs, and a pair of left and right deceleration gear casings supporting the respective left and right drive axles and incorporating the respective deceleration gear trains.
An input shaft projects (rearwardly) outward from the laterally middle (rear) end of the center casing toward a horizontal crankshaft engine in the fore-and-aft direction so as to be drivingly coupled to the engine via a universal joint coupling. A PTO shaft for driving a mower unit projects (forwardly) outward from the center casing opposite to the engine in the fore-and-aft direction. The center casing incorporates a power train including a PTO clutch between the input shaft and the PTO shaft. The power train distributes power between the left and right HSTs on the upstream of the PTO clutch.
Each of the HSTs is disposed in a casing sandwiched between the left or right end of the center casing and each of the left and right deceleration gear casings. In other words, the HST casings are extended laterally outward from the left and right side ends of the center casing, and the deceleration gear casings are mounted onto the laterally distal end of the respective HST casings. Each of the HSTs includes an oil duct plate, and hydraulic pump and motor mounted onto the oil duct plate so as to be fluidly connected to each other via the oil duct plate. The hydraulic pump and motor have respective axial pump shaft and motor shaft extended laterally in parallel to the corresponding drive axle (perpendicular to the input shaft and PTO shaft). Each of the deceleration casings is extended perpendicular to the lateral motor shaft and axle so as to incorporate the deceleration gear train between the corresponding motor shat and axle.
If the HST (for example, its oil duct plate) must undergo repair or maintenance, the deceleration gear casing must be removed from the HST casing incorporating the troubled HST, thereby complicating its repair or maintenance.
Moreover, the transmission is laterally expanded because of the HSTs having lateral pump and motor shafts. Furthermore, each of the deceleration gear casings must be extended in the fore-and-aft direction or vertically so as to incorporate the deceleration gear train interposed between the corresponding lateral motor shaft and axle, thereby distributing the compactness of the transmission.
Another problem of the transmission is that the universal joint coupling is extended between the (rear) end of the center casing and the engine in the fore-and-aft direction. If the heights of the horizontal output shaft of the engine and the horizontal input shaft of the transmission are considerably different from each other and the distance therebetween in the fore-and-aft direction is restricted, the universal joint coupling must be considerably vertically angled (steep) to be interposed between the shafts so as to disturb the efficiency of power transmission, to cause noise and vibration, and to reduce the durability of universal joints.
On the contrary, if the universal joint coupling is extended sufficiently long in the fore-and-aft direction, the above-mentioned problems are solved. However, such a long distance between the transmission and the engine in the fore-and-aft direction shifts the engine toward a front and rear end apart from the transmission so as to unbalance the vehicle (riding lawn mower) in the fore-and-aft direction. Even if the engine is disposed at a position for balancing the vehicle well, the transmission is unexpectedly shifted on the PTO shaft side thereof opposite to the engine in the fore-and-aft direction, thereby restricting the space for the working machine (mower unit).
Furthermore, even if the universal joint coupling is short and disposed almost horizontally, another problem arises that the input shaft of the transmission and the output shaft of the engine must have a short difference of height therebetween so as to reduce the allowed vertically movable range of the mower unit.
Thus, a transmission to be drivingly coupled to a horizontal crankshaft engine via a universal joint coupling is conflictingly requested to have a sufficient space, in which the universal joint coupling can be disposed with its vertical angle reduced as much as possible, and to approach the engine in the fore-and-aft direction.